1. Field of the Invention
The present invention relates to an external force detecting sensor formed by using a semiconductor micro-processing technique or the like.
2. Description of the Related Art
Generally, acceleration sensors and angular velocity sensors are known as external force detecting sensors. Each of these external force detecting sensors is provided with a movable portion which is displaced in accordance with an external force, such as acceleration, angular velocity, or the like applied to the external force detecting sensor. The displacement is electrically detected to obtain an acceleration signal or angular velocity signal. For example, as shown in FIG. 5, an acceleration sensor using a piezoelectric element described in Japanese Unexamined Patent Application Publication No. 10-104263 has a movable portion 1, which includes a weight portion 4 supported on a supporter 2 by beams 3 in the central portion thereof. A supporting substrate 5 and a cap substrate 6 having recesses 5a and 6a, respectively, are mounted to the supporter 2 so as to sandwich the supporter 2 from the top and bottom. In addition, a cavity is formed at the central portion thereof using the recesses 5a and 6a of the supporting substrate 5 and the cap substrate 6, respectively, such that the movable portion 1 can be displaced. In addition, piezoelectric elements 7 are provided on the beams 3, and when acceleration is applied to the weight portion 4 to cause a stress on the beams 3, the piezoelectric elements 7 generate acceleration signals.
However, when the recesses 5a and 6a respectively provided on the supporting substrate 5 and cap substrate 6 are shallow, gaps between the weight portion 4 and a top surface 6b and bottom surface 5b become narrower, and when the weight portion 4 is rapidly displaced, a large phase lag or an output signal occurs. This is due to air damping due to the viscosity of the air sealed in the cavity. As a result, responsiveness of the acceleration sensor deteriorates. Therefore, in the above-described acceleration sensor, in order to eliminate the influence of the air damping, the recesses 5a and 6a of the supporting substrate 5 and cap substrate 6, respectively, are made higher (deeper), and thereby the vertical space in the cavity for the weight portion 4 is increased, thus improving the responsiveness of the acceleration sensor.
The influence of the air damping is the same in an external force detecting sensor which electrostatically detects an external force. Such an external force detecting sensor described in Japanese Unexamined Patent Application Publication No. 2000-22170 is described with reference to FIGS. 6 and 7. By processing a silicon substrate, two weight portions 8 and 9 are combined with supporters 11 and 12 via beams 11a and 12a, respectively, to compose a movable portion 10. The two weight portions 8 and 9 respectively have a plurality of plate-shaped movable interdigitated electrodes 8a and 9a outwardly provided thereon. Fixed portions 13 and 14 are provided at positions respectively opposing the weight portions 8 and 9. The fixed portions 13 and 14 have a plurality of plate-shaped fixed interdigitated electrodes 13a and 14a provided thereon which protruded toward the weight portions 8 and 9, respectively, and are interdigitated with the movable electrodes 8a and 9a, respectively. A frame 15 is provided so as to surround the movable portion 10 and the fixed portions 13 and 14. A functional element composed as described above is supported by a supporting substrate 18 and cap substrate 19 made of Pyrex glass so as to sandwich it from the top and the bottom. In addition, inside the functional element, a cavity is formed by recesses 18a and 19a respectively provided on the supporting substrate 18 and the cap substrate 19, so as to enable displacement of the movable portion 10. On the bottom surface of the recess 18a of the supporting substrate 18, detecting electrodes 16 and 17 are provided beneath the weight portions 8 and 9, respectively, via gaps.
Now, an operation of the external force detecting sensor of the configuration is described when it is used as an angular velocity sensor. When a voltage is applied across the supporters 11 and 12 and the fixed portions 13 and 14, the two weight portions 8 and 9 vibrate in mutually opposing directions due to electrostatic forces exerted between the movable interdigitated electrodes 8a and 9a and the fixed interdigitated electrodes 13a and 14a. In such a vibrating state, when a rotational force is applied to the external force detecting sensor about an axis in a direction connecting the supporters 11 and 12, the two weight portions 8 and 9 experience inverse Coriolis forces in the perpendicular direction. For example, when the weight portion 8 of one side receives a downward Coriolis force, the weight portion 9 of the other side receives an upward Coriolis force, and the two weight portions 8 and 9 vibrate in vector directions respectively determined by the electrostatic force and the Coriolis forces. Due to the vibrations, electrostatic capacitances between the two weight portions 8 and 9 and the detecting electrodes 16 and 17 are differentially altered, and outputs of the two detecting electrodes 16 and 17 are converted into voltages, which are differentially amplified by a differential amplifier to obtain an angular velocity signal.
Now, an operation is described of the external force detecting sensor of the above configuration when it is used as an acceleration sensor. In a state where a D.C. voltage is applied across the supporters 11 and 12, the fixed portions 13 and 14, and the detecting electrodes 16 and 17, when an acceleration is applied to the weight portions 8 and 9, namely from a vector component in a direction connecting the two fixed electrodes, directly opposite acceleration signals are obtained from the two fixed portions 13 and 14. In other words, one of the acceleration signals increases the electrostatic capacitance and the other decreases the electrostatic capacitance. From a vector component in the vertical direction, acceleration signals are obtained from the detecting electrodes 16 and 17. Accordingly, accelerations in two directions can be detected.
In the above-described external force detecting sensor, since the movable portion 10 is displaced in a sealed cavity, the acceleration sensor is strongly influenced by air damping when the movable portion 10 is vertically displaced. In addition, in such a case, when the movable portion 10 is driven to continuously vibrate at a fixed vibration frequency, such as in the angular velocity sensor, air damping exerts an undesirable influence on the operation of the movable portion 10, such as deterioration of the mechanical quality factor of the driving vibration of the movable portion 10, or the like.
Furthermore, when the cap substrate 19 having the recess 19a formed thereon is mounted on the movable portion 10 in a manufacturing process of the external force detecting sensor, a frame 15, the supporters 11 and 12, the fixed portions 13 and 14, and the supporting substrate 18 and the cap substrate 19 are bonded together by an anodic bonding method using a high voltage; this, however, can cause the movable portion 10 to be drawn by a strong electrostatic attraction to the bottom surface of the supporting substrate 18 or the top surface of the cap substrate 19, thus rendering the movable portion 10 inoperable. Accordingly, to avoid this problem, the recesses 18a and 19a of the supporting substrate 18 and the cap substrate 19, respectively, comprising the cavity accommodating the movable portion 10 are preferably formed deep.
However, if the recesses 18a and 19a of the supporting substrate 18 and the cap substrate 19 respectively are formed too deep, the range of vertical movement of the movable portion 10 is increased, and when an external force such as an impact force or the like is applied to the external force detecting sensor from the outside, the movable interdigitated electrodes 8a and 9a of the movable portion 10 exceed the limit of natural return by resiliency of the beams 11a and 12a, thus causing the movable interdigitated electrodes 8a and 9a to ride on the fixed electrodes 13a and 14a, or to jump over the fixed electrodes 13a and 14a and stay there, thus rendering the external force detecting sensor inoperable.
In view of the above-described situations, it is an object of the present invention to provide an external force detecting sensor in which a displacement limit is defined for a movable portion in order to ensure the reliable operation thereof.
In order to solve the above-described problems, an external force detecting sensor according to a first aspect of the present invention comprises a functional element including a supporter, a movable portion having a movable interdigitated electrode, rectangular in cross-section, coupled with the supporter by a beam, and a fixed portion having a fixed interdigitated electrode, rectangular in cross-section, opposing the movable interdigitated electrode via a micro-gap; a supporting substrate for supporting the functional element from one surface side thereof; and a cap substrate mounted on the functional element from the other surface side; wherein a cavity which enables displacement of the movable portion is formed at a portion including the beam and the movable portion, and a height D from the fixed interdigitated electrode to the top surface and bottom surface of the cavity satisfies the following expression where the micro-gap is g, a width of the movable interdigitated electrode is W1, a width of the fixed interdigitated electrode is W2, and a height of the movable interdigitated electrode and fixed interdigitated electrode is h, namely:   D  ≤            h      g        ⁢                  (                  g          +          W1          +          W2                )            .      
By this configuration, the height in the cavity from the fixed interdigitated electrode to the top and bottom surfaces thereof becomes a height at which the movable portion is not influenced by air damping due to a gas in the cavity, and in addition, even if an impact is applied to the external force detecting sensor to cause the movable portion to jump, and as the result, the movable interdigitated electrode falls on the fixed interdigitated electrode, the movable interdigitated electrode securely returns to the standstill position due to the resiliency of the beam.
An external force detecting sensor according to a second aspect of the present invention comprises a functional element including a supporter, a movable portion having a movable interdigitated electrode, rectangular in cross-section, coupled to the supporter via a beam, and a fixed portion having a fixed interdigitated electrode, rectangular in cross-section, opposing the movable interdigitated electrode via a micro-gap; a supporting substrate for supporting the functional element having a first recess which enables displacement of the movable portion provided; and a cap substrate for protecting the functional element having a second recess which enables displacement of the movable portion provided; wherein a height D of the first recess and the second recess satisfies the following expression where the micro-gap is g, a width of the movable interdigitated electrode is W1, a width of the fixed interdigitated electrode is W2, and a height of the movable interdigitated electrode and fixed interdigitated electrode is h, namely:   D  ≤            h      g        ⁢                  (                  g          +          W1          +          W2                )            .      
Accordingly, the cavity in which the movable portion is displaced is composed of the first recess formed on the supporting substrate and the second recess formed on the cap substrate, and the height (depth) of the first recess and the second recess is set at a threshold limit value which promotes the natural return of the movable portion. Accordingly, even if the movable portion receives an impact force, the movable portion naturally returns to the original position, and the external force detecting sensor can continuously operate.
An external force detecting sensor according to a third aspect of the present invention comprises a functional element including a supporter, a movable portion having a movable interdigitated electrode, rectangular in cross-section, coupled to the supporter via a beam, and fixed portion having a fixed interdigitated electrode, rectangular in cross-section, opposing the movable interdigitated electrode via a micro-gap; a supporting substrate for supporting the functional element; and a cap substrate mounted on the functional element from the opposite side of the supporting substrate; wherein a cavity is formed by processing any two of the functional element, the supporting substrate, and the cap substrate at a portion including the beam and the movable portion, and a height D of the recesses and the cavity satisfies the following expression where the micro-gap is g, a width of the movable interdigitated electrode is W1, a width of the fixed interdigitated electrode is W2, and a height of the movable interdigitated electrode and fixed interdigitated electrode is h, namely:   D  ≤            h      g        ⁢                  (                  g          +          W1          +          W2                )            .      
Since the cavity is a space which enables displacement of the movable portion, the cavity can be formed on the functional element itself when the functional element is processed. Therefore, when either of the supporting substrate or the cap substrate, including the functional element, is processed, spaces are formed above and beneath the movable portion. Even in this case, since the movable portion functions sufficiently, and the supporting substrate and cap substrate work as stoppers even if the movable portion jumps due to an impact force, the movable interdigitated electrode never remains riding on the fixed interdigitated electrode.
An external force detecting sensor according to a fourth aspect of the present invention comprises a functional element including a fixed portion, a supporter, and a movable portion coupled to the supporter by a beam; a supporting substrate for supporting the functional element; and a cap substrate for protecting the functional element; wherein the supporting substrate and the cap substrate are arranged so as to sandwich the functional element from both surfaces of the functional element while forming a cavity, which enables displacement of the movable portion, at a portion including the beam and movable portion, the movable interdigitated electrode, rectangular in cross-section, is provided on the movable portion and the fixed interdigitated electrode, rectangular in cross-section, which is provided with the movable interdigitated electrode via a common micro-gap is provided on the fixed portion, and the height from the movable portion to the top surface and the bottom surface of the cavity is set to be the same as or lower than a height of a surface of the movable interdigitated electrode at a far side from the fixed interdigitated electrode when the movable interdigitated electrode is moved along a straight line passing through peaks of corners diagonally positioned on opposing sides of the movable interdigitated electrode and the fixed interdigitated electrode which are adjacent across the micro-gap, and when surfaces of the movable interdigitated electrode and the fixed interdigitated electrode on sides which are not opposed are positioned in the same plane.
According to the present invention, independence of the size of the impact force applied to the external force detecting sensor, since the movable portion inevitably collides with the top or bottom surface of the cavity, the top or bottom surface works as a stopper, thereby even if the movable portion collides with the top or bottom surface and the movable interdigitated electrode falls on the fixed interdigitated electrode, the movable interdigitated electrode is securely drawn back to the original standstill position, thus facilitating continuous use of the external force detecting sensor.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.